54 



THE ANIMAL KINGDOM. 



a longer or shorter peduncle (imago of Lepidoptera, Diptera, many Neurop- 

 tera). The stomodaeal section is usually straight and traverses the thorax. 

 The mesenteron and proctodaeum lie in the abdomen in the imago and 

 are more or less convoluted, the coils being held in position by tracheae, 

 fatty tissue, and nerves. The mesenteron or chylific stomach varies in 

 length. Its anterior extremity may develope a few caecal tubes (many 

 Orthoptera^ Plecopterd). Its epithelium contains glandular cells, and its 

 outer surface is sometimes beset with villiform glands (some Coleoptera). 

 The proctodaeum does not communicate with the mesenteron in the larvae 

 of aculeate Hymenoptera, some Neuroptera (Myrmeleo, Hemorobiidae\ and 

 Pujpipara among Diptera. In the Neuroptera named it forms a silk gland. 

 It is generally of some length and its calibre varies in different regions. 

 The short posterior dilated section or rectum is generally, especially in the 

 imago, provided with 1-6 longitudinal ridges richly supplied with tracheae. 

 These ridges become of functional respiratory importance in the larval 

 Odonata, in which water is taken into and expelled from the rectum 

 rhythmically. The anus is terminal in the last abdominal somite. 



Excretory organs or Malpighian tubes open into the commencement 

 of the proctodaeum, rarely into the end of the mesenteron as in Thysanura, 

 TermeS) &c. They arise from the proctodaeum as one or two pairs of 

 outgrowths, but their number may be subsequently increased by budding. 

 The full number present may be small (2-8) or large (30-50 or more, e. g. 

 150 in Apis}. Hence Insecta may be designated as Oligo- or Poly-nephria. 

 They may branch, and they open into the proctodaeum separately or 

 united in bundles with a common aperture. 



The coelome is more or less filled by the viscera and fat-body when 

 present. The latter is always plentiful in the larva, and persists to a 

 variable degree in the imago. It is a tissue composed of fat-containing 

 cells richly supplied with tracheae. The blood is colourless or yellowish, 

 tinged with green in vegetable-feeders, or sometimes reddish. In the 

 larva of Chironomus (Diptera) it is deeply coloured with Haemoglobin. 

 The blood corpuscles are amoeboid \ The heart is placed dorsally in the 

 abdomen. It consists in the imago usually of eight chambers with lateral 

 valved ostia. The anterior end gives off an aortic trunk which extends to 

 the head. The posterior end is closed, and but rarely gives off vessels. 

 The organ is suspended by muscular filaments to the terga, and lies in 

 a pericardial sinus limited ventrally by the alary muscles, the contraction 



1 Von Wielowicjski distinguishes three types of blood-cells in Insecta. (i) Fatty cells, con- 

 stituting the fat body ; they are grouped together, are rarely bi- or multi-nuclear, and their pro- 

 toplasm is filled with fat drops. (2) Oenocyths, multi-nucleate cells with slightly coloured proto- 

 plasm and containing but few granules, tied to the smallest tracheal capillaries. (3) Pericardial 

 dells, which differ much in character but are known by their position in the pericardial system. 

 Z. W. Z. xliii. 1886. 



